BIOLOGY 457/657 PHYSIOLOGY OF MARINE & ESTUARINE ANIMALS February 18, 2004 Osmoregulation in Marine Vertebrates

A COMPARISON OF VERTEBRATE & INVERTEBRATE OSMOREGULATION

VERTEBRATE OSMOREGULATION: OVERVIEW

VERTEBRATE OSMOREGULATION Two Major Vertebrate Groups: Fish & Amphibians (evolved in water) Reptiles, Birds, & Mammals (evolved in air – to these animals, seawater is a “desert”) It is thought that the first modern vertebrates evolved in freshwater, because their solute levels are low. However, some primitive fishes have solute concentrations very similar to seawater. Modern fishes must have returned to the ocean from freshwater. Some fish alternate between habitats: Anadromous – breed in FW, live as adults in SW Catadromous – breed in SW, live as adults in estuaries

OSMOREGULATION IN FISHES

FISHES I: CYCLOSTOMES Cyclostomes (Class: Agnatha) are jawless, primitive fish. Hagfishes (marine) are isosmotic, using FAA to adjust Π. Lampreys (freshwater or anadromous) are like teleosts, with excellent Na + transport systems K t (Na + ) = 0.13 mM in gill tissue.

FISHES II: ELASMOBRANCHS Elasmobranchs (Class: Condrichthyes; sharks & rays) are generally marine, with a few freshwater species (e.g. Amazon ray, bull sharks of Lake Nicaragua) Their inorganic solute concentrations are similar to those of teleosts (bony fishes). The deficit is made up using organic solutes.

ELASMOBRANCHS (2) The shark kidney actually reabsorbs urea, back into the blood. Shark proteins are highly tolerant of elevated [urea]. Total osmoticity is usually slightly > 1000 mOsm, so water continually enters by osmosis (for metabolism and urine formation) Freshwater elasmobranchs excrete urea, reduce drinking, and increase salt uptake (perhaps by the gills or digestive tract).

CLASS: CROSSOPTERYGII The lobe-fin fishes, represented today only by the coelacanth (genus Latimeria), are ancestral to all terrestrial vertebrates. Physiologically (with regard to osmoregulation), they are like the elasmobranchs.

CLASS: TELEOSTII (Bony Fishes) All teleosts have similar blood osmolality, about 25% to 40% of seawater. Many species occupy both fresh and salt water. The Osmotic Problem:

MARINE FISHES Are hyposmotic: LOSE water through the gills, body surface, and in wastes. DRINK seawater (a flounder drinks about 1% of its mass per hour) Are hypoionic: GAIN salts across gills, body surface, and in food. EXCRETE salts (Na +, Cl - ) across gills. Other ions (K +, Ca ++, Mg ++, SO 4 = ) are excreted by the kidney in the hypotonic urine, by tubular secretion.

MARINE FISHES Chloride Cells: Sites of ion transport in fish gills. Cations (Na +, K + ) are cotransported with Cl - ions at the basal surface. Cl - passive passes out of anion channels at the apical surface, and Na follows through leaky tight junctions between cells.

FRESHWATER FISHES Are hyperosmotic, and gain water in food, across the gill, and across the body surface. Excrete a dilute, copious urine (up to 400 ml/kg/day). Are hyperionic, and lose salts in urine and by diffusion. Import salts in diet and by active transport at the gill.

EURYHALINE FISHES Example: Xiphister (blenny) (Must shift between modes)

CLASS AMPHIBIA Most amphibians are freshwater or terrestrial, but a few are euryhaline. Most are ammonicotelic, but some euryhaline species are ureotelic. Euryhaline species actually osmoconform at elevated salinities, using ions or urea.

CLASS REPTILIA Reptiles are essentially terrestrial, but some species live in freshwater, marine, or estuarine habitats. All are uricotelic. However, the estuarine diamondback terrapin (Malaclemys) actually uses urea as an osmotic effector! Marine species (turtles, crocodiles, snakes, iguana) produce an isotonic urine and excrete salt using extrarenal salt glands.

CLASS REPTILIA SALT GLANDS Marine Iguana – nasal gland, sneezes Snakes, Turtles - supraorbital gland, drain secretions into the orbit (turtles) or oral cavity (snakes) Crocodiles – sublingual salt glands, drain into oral cavity

CLASS AVES Like reptiles, birds are essentially terrestrial animals, but have many successful marine species (and are more diverse than any other air-breathing vertebrate). All reproduce on land. Urine is hypotonic or isotonic. All are uricotelic. Marine birds have a large extrarenal pair of salt glands external to the skull, producing a concentrated solution of NaCl.

CLASS AVES Salt-gland secretion by a herring gull. The gland uses a countercurrent mechanism to concentrate salt in the secretion, and can transport salt at up to 20x the rate (per gram of tissue) as a human kidney.

CLASS MAMMALIA All marine mammals are secondarily evolved from terrestrial ancestors. They form a diverse group of animals: pinnipeds, sirenians, otters, and cetaceans. Many marine mammals gain water from their food (e.g. other vertebrates, teleosts). Their kidneys can produce urine that is hypertonic NOT ONLY to the blood, BUT ALSO to seawater.

CLASS MAMMALIA